Tcp sequencing and IP fragmentation

Posted on 2004-09-06
Last Modified: 2013-11-29
I know TCP segments are numbered (with sequence numbers) for reassembly/error recovery reasons.  
How do TCP segments and IP fragmentation correlate?  If the sender has an IP datagram that exceeds the layer 2's MTU, it fragments the datagrams and each datagram gets sent as separate frames across the wire.

Now, what is responsible for reassembly on the other end?   I know that with packet fragmentation, a packet id is defined and the "fragment" flag is turned on.  What is TCP's job in the reassembly of fragged IP datagrams?  Arent the fragmented packets ID'd anyway?
Question by:dissolved
  • 3
  • 3

Accepted Solution

LimeSMJ earned 500 total points
ID: 11991697
The Internet layer (IP protocol) within the TCP/IP architecture is responsible for reassmebly of a fragmented packet.

If a packet hits a network where the MTU is lower than the originator, the packet is fragmented to allow for transmission.  The fragmentation is done usually by the routers along the way so the sender does not know of the packet manipulation (sender won't decrease packet size if MTU is smaller).  Since the sender is only sending sequenced packet numbers (not caring about fragmentation), the fragmented packets need to have their own sequencing flags added in by the router - so that the IP layer can defragment at the receiving end.

TCP's job is provide the transmission link... IP helps negotiate the data packets along the way.


Author Comment

ID: 11991843
Cant fragmentation occur between the network layer and data link layer as well?
I heard that if an IP datagram is handed to the data link layer, it will be fragmented if it is to big. And each packet leaving the network will have the fragment flag turned on.


Author Comment

ID: 11991850
Also, cant the receiver send a ICMP error back to the sender saying "needs fragmentation" or something like that?
Control application downtime with dependency maps

Visualize the interdependencies between application components better with Applications Manager's automated application discovery and dependency mapping feature. Resolve performance issues faster by quickly isolating problematic components.


Expert Comment

ID: 11992029
The ICMP request for fragmentation is generated by a router if the packet received has the "do not fragment" bit set and cannot be forwarded because the next hop MTU is too small for the packet (fragmenting is not possible to make the MTU smaller, so this error occurs).  The actual destination computer would never send an ICMP request for fragmentation.

Expert Comment

ID: 11992101
I am not sure what you meant by :

"...IP datagram is handed to the data link layer, it will be fragmented if it is to big..."

The IP datagram is the packet itself... the sent payload.  If the sending computer knows of potential MTU problems, it may allow the packet to be fragmented (thus preventing any transmission delays).  Unless there is a stack misconfiguration, there is no way for a datagram to be internally fragmented between the OSI layers.

Author Comment

ID: 11992105

Featured Post

Windows Server 2016: All you need to know

Learn about Hyper-V features that increase functionality and usability of Microsoft Windows Server 2016. Also, throughout this eBook, you’ll find some basic PowerShell examples that will help you leverage the scripts in your environments!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Suggested Solutions

If you're not part of the solution, you're part of the problem.   Tips on how to secure IoT devices, even the dumbest ones, so they can't be used as part of a DDoS botnet.  Use PRTG Network Monitor as one of the building blocks, to detect unusual…
Most of the applications these days are on Cloud. Cloud is ubiquitous with many service providers in the market. Since it has many benefits such as cost reduction, software updates, remote access, disaster recovery and much more.
Internet Business Fax to Email Made Easy - With  eFax Corporate (, you'll receive a dedicated online fax number, which is used the same way as a typical analog fax number. You'll receive secure faxes in your email, f…
In this tutorial you'll learn about bandwidth monitoring with flows and packet sniffing with our network monitoring solution PRTG Network Monitor ( If you're interested in additional methods for monitoring bandwidt…

911 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

23 Experts available now in Live!

Get 1:1 Help Now